tests/fixedbugs: regression test for issue 100 (#129)

Adds a regression test based on klauspost/compress#186. This necessitated some related changes:

* Mark "RET" as a terminal instruction
* printer refactor to maintain compatibility with asmfmt
* Tweaks to other regression tests to ensure they are run correctly in CI

Updates #100 #65 #8

tests/fixedbugs/issue65/doc.go

@@ -0,0 +1,9 @@
+// Package issue65 is a regression test for a bug involving casting physical registers.
+//
+// Regression test for a bug where casting a physical register would give the
+// error "non physical register found".
+//
+// See: https://github.com/mmcloughlin/avo/issues/65#issuecomment-576850145
+package issue65
+
+//go:generate go run asm.go -out issue65.s -stubs stub.go